Measurement devices—also denoted as measuring instruments or measuring apparatus—can be used to measure physical parameters or signals. Measurement devices include a variety of different types of measurement devices such as oscilloscopes, signal analyzers, network analyzers or testing devices. Measurement devices can be used in production facilities for providing services, in particular repair and maintenance services. Further, measurement devices are used in research or investigating devices under test. Measurement devices require calibration and/or recalibration of components.
Vector network analyzers VNA are designed to accurately characterize the linear behaviour of a device under test, DUT, by evaluating the phase and magnitude of incident and reflected signal waves. By measuring a phase and magnitude of these waves it is possible to determine a wide range of device characteristics comprising impedance, return loss, insertion loss and even group delay. The accuracy with which a vector network analyzer can determine a linear behaviour of a device under test is fundamentally determined by the accuracy with which the vector network analyzer can measure the phase and magnitude relationship of incident and reflected waves.
Although it is possible to calibrate certain elements of the vector network analyzer at manufacturing time such as source power and receiver accuracy, much of the calibration information required to perform an accurate measurement depends on the measurement setup or test arrangement.
FIG. 1 illustrates a test arrangement where a device under test DUT is connected to a test port P of a measurement apparatus such as a vector network analyzer VNA.
In the test arrangement as illustrated in FIG. 1, it is well-known that the open circuit has an infinite impedance and that the reflected wave does have the same magnitude as the incident wave and is in-phase with the incident wave. However, the length of the cable between the vector network analyzer port P and the open circuit does affect the phase as measured by the vector network analyzer VNA. Although calibration can be useful to verify the accuracy of certain elements of a vector network analyzer such as the power of the stimulus and the frequency response of the receivers, other characteristics related to the measurement setup, such as cable length, are impossible to account for during calibration. As a result, a user calibration is necessary that not only accounts for variables due to measurement configuration but which can also account for instrument variations, for instance with respect to temperature and frequency changes.
Systematic sources of error can comprise port match, directivity, frequency, response and isolation. These errors are systematic sources of error because they systematically affect the measurement at all times. The impact of systematic errors on a measurement result can largely be removed through calibration.
When calibrating a measurement apparatus such as a vector network analyzer VNA, engineers can utilize calibration and verification tools. A calibration kit can comprise different calibration components. A manufacturer of a measurement apparatus can provide a range of calibration kits for various connector types and for both manual and automatic calibration. Calibration kits comprise calibration elements or calibration components. These calibration components can comprise calibration standards that have been carefully characterized based on the mechanical dimensions or based on measurements using a so-called golden network analyzer that has been calibrated using traceable calibration standards.
A calibration kit also contains precise information about the behaviour of each calibration component and the measurement apparatus can use this information as part of the calibration routine.
FIG. 2 illustrates a conventional measurement apparatus such as a vector network analyzer VNA having a test port P to which two calibration components CC are connected during a calibration process. The calibration components or calibration standards used for calibration can be for instance an OPEN calibration standard, a SHORT calibration standard, a THROUGH calibration standard or a REFLECT calibration standard. To use a calibration standard or a calibration component for a VNA it is necessary that the VNA is informed about the characteristics of the respective calibration standard element. This data can comprise for instance coefficients for modelling effects of the capacity or geometric parameters such as cable length or S-parameter data. In a conventional VNA, the VNA has access to the characteristic data of the calibration standard components by means of a data carrier, a data sheet or the characteristic data is defined in a firmware of the measurement apparatus. FIG. 2 illustrates a memory stick such as an USB stick which is inserted into a data interface of the measurement apparatus. In the illustrated example, the memory stick USB stores the characteristic data of the calibration standard components CC used for the calibration of the measurement apparatus. However, this conventional way of providing the characteristic data of a measurement accessory device such as a calibration standard component has several disadvantages. It can happen that a user or engineer performing the calibration uses the wrong memory stick carrying calibration data of other calibration standard components not used in the calibration setup. Further, it can happen that a data carrier such as a memory stick storing the characteristic data of a measurement accessory device cannot be found and has to be searched for or gets completely lost. In a conventional calibration set-up as illustrated in FIG. 2, there is no security check whether the measurement apparatus uses the correct characteristic data for the respective calibration standard components. A not-experienced user or engineer may not recognize that the characteristic data does not match the used calibration standard components which leads to erroneous measurement results when measuring a device under test DUT by the measurement apparatus.
Accordingly, there is the need to overcome the above-mentioned disadvantages and to facilitate a calibration and/or measurement process.